Polymerizable sulfonate ionic liquids and liquid polymers therefrom

ABSTRACT

Disclosed is a new ionic liquid monomer salt and methods of is synthesis and polymerization. The ionic liquid monomer salt is prepared by mixing equimolar amounts of an amine, such as tris[2-(2-methoxyethoxy)-ethyl]amine and an acid functionalized polymerizable monomer, such as 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), which is stirred at ambient temperature until salt formation is complete. Also disclosed is a new ionic liquid polymer salts and method for making the same. The synthesis of AMPS-ammonium salt polymer is accomplished by adding 2,2′-azobisisobutyronitrile (AIBN) to the ionic liquid monomer salt and heating the homogeneous melt at 70° C. for 18 hr.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional of application Ser. No. 11/894,639filed on Aug. 17, 2007. Application Ser. No. 11/894,639 claims thebenefit of U.S. Provisional Application 60/822,772 filed on Aug. 18,2006, both of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

Ionic liquids are salts composed of cationic and anionic componentswhose structures impart a sub-room temperature melting point or glasstransition to the resulting material. A liquid character is associatedwith ions that have very weak tendencies to coordinate toward oppositelycharged ions (e.g. charge delocalized or sterically shielded), withsubstituents that have weak intermolecular forces (e.g. fluorocarbons,alkanes, silicones) and with a structural symmetry that is not conduciveto efficient molecular packing. Most ionic liquids are organic salts.The cationic component is usually organic in nature (e.g.alkyl-substituted ammonium, phosphonium, imidazolium and pyridinium),and the anionic component is most often inorganic (e.g. nitrate,sulfate, thiocyanate, halide, tetrafluoroborate, hexafluorophosphate,etc.) but may also be organic (e.g. tosylate, alkylsulfates,fluoroalkylsulfates, alkylcarboxylates, fluoroalkylcarboxylates, etc.).These liquid materials have unique properties (immeasurably lowvolatility, non-flammability, very high polarity and solvatingcharacteristics, high ionic conductivity, and a wide electrochemicalpotential window). There is currently much interest in their use assolvents for a large variety of reactions and in sampling for chemicalanalysis. See Zhao et al., J. Chem. Technol. Biotechnol. 2005, 80, 1089and Welton, Chem. Rev. 1999, 99, 2071.

A more unique form of ionic liquid is based on a system where one of thetwo charged components is a polymer. As such, each repeat unitincorporates the same ionic site. There are very few reports in theliterature of such systems. They are mostly based on the imidazolium ionfunctionalized with a polymerizable vinyl, acrylic or styryl moiety andhave the physical form of a glass or sticky rubber. Reports includeimidazolium polymers. See Washiro, et al. Polymer, 2004, 45, 1577; Ding,et al., J. Poly. Sci. Part A, 2004, 42, 5794; Tang, et al. J. Poly. Sci.Part A, 2005, 43, 5477; and Nakajima, et al., Polymer, 2005, 46, 11499and alkali metal sulfonate polymers, see Ogihara, et al., Electrochim.Acta, 2004, 49, 1797 and Binnemans, Chem. Rev., 2005, 105, 4148. Thesepolymers are prepared from monomers which themselves may or may not beionic liquids. In the case of cationic imidazolium polymers, certainimidazolium monomer ionic liquids will yield the corresponding polymerionic liquid if appropriate substitution is made on the imidazole ring;otherwise, a glassy solid is obtained. An appropriate substitutionrelates to the addition of a sufficient number and/or sufficient size ofalkyl groups to the ring. In contrast, the anionic-form of a polymerionic liquid has yet to be prepared directly from its analogous monomerionic liquid. For instance, the high melting sulfonate monomer solid(usually an alkali metal salt) is first polymerized in solution followedby substitution with an appropriate cationic counter ion. Solventremoval is necessary to generate the anionic polymer liquid, stillretaining the alkali metal ion.

Polymerizable ionic liquids and their actuation in an electric field area combination of material and properties with unique potential todisplay structural and fluid dynamics above that found for smallmolecule ionic liquids. Small molecule ionic liquids are generallymonovalent organic salts with melting points or glass transitions belowroom temperature. They derive their liquid character from a selection ofionic structures which have very weak tendencies to coordinate withoppositely charged ions, low intermolecular forces and low symmetry.Their properties (immeasurably low volatility, non-flammability, veryhigh polarity and solvating characteristics, high ionic conductivity,and a wide electrochemical potential window) are of substantial interestparticularly with regard to applications as green solvents, analyticalextraction solvents, and electrochemical supporting media. Very recentlyit has been reported that water immiscible ionic liquids displaysignificant electrowetting characteristics with an interestingdependence on the size of the cationic and anionic components. SeeRalston, et al., J. Am. Chem. Soc., 2006, 126, 3098. Ionic liquidsthemselves provide an opportunity of producing a more stable actuatingmedium, eliminating such issues as solvent evaporation and degradationdue to electrolysis, typically found in aqueous based electric fieldinduced actuators.

In an ionic liquid polymer system the cationic or anionic centers areconstrained to repeat units in the polymer chain. As such, any molecularflow or diffusion requires a concerted motion of as many ionic centersas there are charged repeat units in the polymer chain. When subjectedto an electric field, a polymeric system may respond in an enhanced orretarded manner relative to a small molecule ionic liquid, depending onwhether the covalent linkage of cationic or anionic repeat unitsresponds as a more highly charged single molecule or whether itsmacromolecular size inhibits molecular motion needed for a response.

BRIEF SUMMARY OF THE INVENTION

Disclosed is a new ionic liquid monomer salt and methods of is synthesisand polymerization. The ionic liquid monomer salt is prepared by mixingequimolar amounts of an amine, such astris[2-(2-methoxyethoxy)-ethyl]amine and an acid functionalizedpolymerizable monomer, such as 2-acrylamido-2-methyl-1-propanesulfonicacid (AMPS), which is stirred at ambient temperature until saltformation is complete. Also disclosed is a new ionic liquid polymersalts and method for making the same. The synthesis of AMPS-ammoniumsalt polymer is accomplished by adding 2,2′-azobisisobutyronitrile(AIBN) to the ionic liquid monomer salt and heating the homogeneous meltat 70° C. for 18 hr.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a differential scanning calorimetry comparison of oxyethyleneamine, AMPS oxyethylene ammonium salt monomer, and polymer illustratinglow Tg upon salt formation in the monomer and polymer;

FIG. 2 is a depiction of an electrowetting actuation electrode setupwith (right) and without (left) an induced electric field;

FIG. 3 shows electrowetting curves of monomer (left) and polymer;

FIG. 4 shows electrowetting AMPS oxyethylene ammonium salt polymer asvoltage increase from 0 V (left) to 157 V (right).

DETAILED DESCRIPTION OF THE INVENTION

Ionic liquid polymers and their actuation in an electric field are acombination of material and properties with unique potential to displaystructural and fluid dynamics above that found in small molecule ionicliquids. These structure and property dynamics are directly dependentupon an incorporation of a large number of positive or negative chargeson the same polymer molecule while maintaining a liquid character underambient conditions and a large temperature range where the polymer is aliquid or readily deformed viscoelastic solid. The strategy used in thisinvention for preparation of such polymers is to select acidfunctionalized monomer and organic base components that form acid-basesalts that are liquids at room temperature and polymerize to formpolymers that are also liquids at room temperature or, if lightlycrosslinked, form very easily deformed viscoelastic solids. The acidfunctionality is selected as one having a strong acidity and one that isreadily attached to polymerizable monomers. The sulfonic acid isparticularly attractive although phosphoryl or carboxylic acids couldalso be made to serve this function. Functionalization of an acid grouponto a polymerizable monomer such as a styrene, acrylate, olefin, vinylether, or acrylamide results in a relatively high temperature meltingmonomer. The 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS)monomer, used in the example, has a 192° C. melting point. Othercommercially available monomers appropriate for this invention wouldinclude styrene sulfonic acid and vinyl sulfonic acid. Sulfonic acidfunctionalized monomers are the preferred class acid functionalizedmonomer with the AMPS monomer being the most preferred.

An organic base which has a very low melting point or glass temperatureis selected so that it will depress the corresponding melting point orglass transition of the salt it forms with the acid monomer to an extentthat its liquid range extends well below room temperature. For example,the organic base tris[2-(2-methoxyethoxy)-ethyl]amine, there are twooxyethylene groups in each amine substituent. These substituents causethe amine to have a very low glass transition, −104° C., and it is thislow glass transition that causes the salt it forms with the AMPS monomerto also to have a low glass transition, −57° C. Other amines withsimilar characteristics (e.g. different numbers or mixed numbers ofoxyethylene units) or with different substituents also correlating withlow glass transitions (e.g. dimethylsiloxane and fluoromethylene) couldwork comparably. The amine base is the preferred organic base with theoxyethylene functionalized tertiary amine base being the most preferred.A physical mixing of the sulfonic acid functionalized monomer with theamine base generates the ionic liquid monomer salt which is thenpolymerized to form the corresponding ionic liquid polymer salt.Although not required, addition of a small amount of volatile solventwill promote a more rapid dissolution of the sulfonic acidfunctionalized monomer in the amine base; including, but not limited tomethanol and ethylacetate.

A small quantity of polymerization catalyst is added to ionic liquidmonomer salt to effect this transformation. The preferred catalysts arethose that initiate free radical polymerization, and those that havehigh initiator efficiencies and solubility in the ionic liquid monomersalt are the most preferred. In cases where initiators need solubility,enhancement a small quantity of volatile, readily removable solvent suchas methanol or ethyl acetate can be used to disperse the initiator inthe monomer melt. The quantity of catalyst and the polymerizationconditions used vary according to the specific characteristics of thecatalyst and polymer molecular weight desired. A preferred catalyst,such as 2,2′-azobisisobutyronitrile, will typically be used in aquantity corresponding to a range of monomer:initiator ratio range of50:1 to 1000:1 and under of temperature and reaction conditions of 50 to110° C. and 3 to 24 hours respectively. An important feature of themonomer salt and of the ionic liquid polymer product is that they be ahomogeneous melt throughout all stages of polymerization conversion.This feature results in the polymerization approaching quantitativeconversions without an added solvent which would have to be subsequentlyseparated. That the polymer product is also an ionic liquid qualifies itfor unique electric field driven actuations such as physical shapechanges, spreading on surfaces and droplet movement.

Provided are a composition of matter identified as an ionic liquidpolymer salt, composed of an organic sulfonate repeat unit and acationic organic counter ion and a general synthetic procedures forpreparation of this class of compounds from ionic liquid monomers.Interest in such compositions of matter is driven primarily by electricfield actuations in the form of electrowetting or Maxwell stressdeformation of such materials. More specifically, an ionic liquidpolymer system wherein a low molecular weight counter ion promotes anionic liquid character in both the monomer salts and polymer salts andan observation of electrowetting where the polymer salt displays adistinctive effect relative to that for the monomer salt is disclosed.Disclosed is the preparation of a ionic liquid monomer salt, itssolvent-free or solvent-assisted polymerization to the correspondingionic liquid polymer salt, and an electrowetting wetting property thatexceeds that of low molecular weight ionic liquids.

This new composition of matter is prepared utilizing a simple process;yet, retains much versatility for property development. By virtue ofhaving a large number of ionic charges incorporated into a singlepolymer molecule which has liquid-like characteristics, this materialhas the capability of a unique response to an applied electric field.Advantages and new features reside in the accompanying properties andthe applications they can support. Compared with low molecular weightionic liquids, this material is capable of a large and polaritydependent response to an electric field. Applications that can takeadvantage of this property are actuators. In electronics it could findapplication as a dielectric material in a capacitor or perhaps as anelectromigrating substance. Independent of the presence of an electricfield, there are surface application possibilities such as a primer toimprove surface wetting characteristics of low energy surface to acceptpaints and coatings of low polarity, as a biocidal treatment forsurfaces, and as an adsorbent for opposite charged polyelectrolytes andpossibly simple ionic species in chromatography columns. In solutionthis material could conceivably be used as a phase transfer agent or asa sequestrate for ionic substances. As a precursor to a polymer withdifferent mechanical properties than a liquid, it could be converted toan elastomer or a flexible plastic and still retain its high ioniccharacter.

In one embodiment, an ionic liquid monomer salt is provided, having thegeneral formula:

wherein:

A represents H or CH₃;

X represents —COY—, -p-C₆H₄—, -o-C₆H₄—, -m-C₆H₄—, —O—, or —CH₂—;

Y represents: —O(CH₂)_(n)—, where n=1 to 4, or

-   -   —(OCH₂CH₂)_(n) where n=1 to 6, or    -   —NHCH(CH₃)—, —NHC(CH₃)₂—, —N(C₁₂H₂₅)CH₂CH₂—,    -   —NHC(CH₃)CH₂—, or —NHCH(C₆H₅)CH₂—,

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12, or    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

Another embodiment of an ionic liquid monomer salt is presented, havingthe formula:

In another embodiment, an ionic liquid monomer salt is presented, havingthe formula:

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12, or    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

In another embodiment, an ionic liquid polymer salt is presented, havingthe formula:

wherein m>1;

A represents H or CH₃;

X represents —COY—, -p-C₆H₄—, -o—C₆H₄—, -m-C₆H₄—, —O—, or —CH₂—;

Y represents:

-   -   —O(CH₂)_(n)— where n=1 to 4, or    -   —(OCH₂CH₂)_(n) where n=1 to 6, or    -   —NHCH(CH₃)—, —NHC(CH₃)₂—, —N(C₁₂H₂₅)CH₂CH₂—    -   —NHC(CH₃)CH₂—, or —NHCH(C₆H₅)CH₂—,

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12, or    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

In another embodiment, an ionic liquid polymer salt is presented, havingthe formula:

In another embodiment, an ionic liquid polymer salt is presented, havingthe formula:

wherein m>1;

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12,    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4,    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6,    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

In another embodiment, a method for making an ionic liquid monomer saltis presented. An equimolar amount of a sulfonic acid functionalizedmonomer having the general formula:

wherein A represents H or CH₃,

-   -   X represents —COY—, -p-C₆H₄—, -o-C₆H₄—, -m-C₆H₄—, —O—, or —CH₂—,    -   Y represents —O(CH₂)_(n)— where n=1 to 4, or        -   —(OCH₂CH₂)_(n) where n=1 to 6, or        -   —NHCH(CH₃)—, —NHC(CH₃)₂—, —N(C₁₂H₂₅)CH₂CH₂—,        -   —NHC(CH₃)CH₂—, or —NHCH(C₆H₅)CH₂—,

is mixed with an organic amine base having the general formula:

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12, or    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

The mixture is stirred under an inert atmosphere until the sulfonic acidfunctionalized monomer is dissolved, wherein the dissolution completesformation of the ionic liquid monomer salt.

In another embodiment, a method for making an ionic liquid monomer saltis presented. An equimolar amount of a sulfonic acid functionalizedmonomer having the formula:

is mixed with an organic amine base having the formula:

The mixture is stirred under an inert atmosphere until the sulfonic acidfunctionalized monomer is dissolved, wherein the dissolution completesformation of the ionic liquid monomer salt. Optionally, the ionic liquidmonomer salt is in solvent methanol.

In another embodiment, a method for making an ionic liquid monomer saltis presented. An equimolar amount of a sulfonic acid functionalizedmonomer having the formula:

is mixed with the organic amine base having the general formula:

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12, or    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

The mixture is stirred under an inert atmosphere until said sulfonicacid functionalized monomer is dissolved, wherein the dissolutioncompletes formation of the ionic liquid monomer salt.

In another embodiment, a method for making ionic liquid polymer salts ispresented. A free radical polymerization catalyst is added to an ionicliquid monomer salt having the general formula:

wherein A represents H or CH₃

-   -   X represents —COY—, -p-C₆H₄—, -o-C₆H₄—, -m-C₆H₄—, —O—, or —CH₂—    -   Y represents —O(CH₂)_(n)— where n=1 to 4, or        -   —(OCH₂CH₂)_(n) where n=1 to 6,        -   —NHCH(CH₃)—, —NHC(CH₃)₂—, —N(C₁₂H₂₅)CH₂CH₂—,        -   —NHC(CH₃)CH₂—, or —NHCH(C₆H₅)CH₂—,

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12, or    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4,    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6,    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8

The mixture is stirred under an inert atmosphere with applicationcatalyst-specific polymerization conditions.

In another embodiment, a method for making ionic liquid polymer salts ispresented. A free radical polymerization catalyst is added to an ionicliquid monomer salt having the formula:

The mixture is stirred under an inert atmosphere with applicationcatalyst-specific polymerization conditions. Optionally, the freeradical polymerization can be conducted in solvent methanol. Preferably,the free radical polymerization catalyst is 2,2′-azobisisobutyronitrileand the monomer to free radical polymerization catalyst ratio is 100:1.The inert atmosphere is preferably a nitrogen atmosphere. Thecatalyst-specific polymerization conditions include heating the mixtureto 70° C. and stiffing and heating steps are conducted for about 18hours.

In another embodiment, a method for making ionic liquid polymer salts ispresented. A free radical polymerization catalyst to said ionic liquidmonomer salt claim 3 having the general formula:

wherein each R, R′, and R″ can be the same group, or R can differ fromR′, while R′ is the same group as R″, or R, R′, and R″ may be alldifferent groups; wherein

R, R′, R″ each independently represented by one of the following:

-   -   —H, or    -   —(CH₂)_(n)CH₃ where n=1 to 12,    -   —(CH₂CH₂O)_(n)CH₃ where n=1 to 4,    -   —(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6,    -   —(CF₂)_(n)CF₃, —CH₂(CF₂)_(n)CF₃ where n=1 to 8.

The mixture is stirred under an inert atmosphere with applicationcatalyst-specific polymerization conditions.

This invention converts 1:1 mixture of a monomer having a polymerizablecarbon-carbon double bond and a sulfonic acid or sulfonate group (suchas 2-Acrylamido-2-methyl-1-propanesulfonic acid, AMPS, mp 192° C.) withan amine base (such as tris[2-(2-methoxyethoxy)-ethyl]amine) to an ionicliquid monomer salt that is polymerized to an ionic liquid polymer saltin the absence or presence of solvent. To convert the sulfonic acidmonomer to a salt with a melting point or glass transition below roomtemperature, it complexed with an an amine is added. This amine isfunctionalized with large flexible substituent structures that depress asalt solidification temperature and inhibit coordination betweenoppositely charged species. Typical amine substituent structures involveoxyethylene oligomers, silicone oligomers, and fluorocarbon chains.

The synthesis is a two-step procedure as depicted below using the AMPSmonomer and a tertiary amine with oxyethylene appendages of seven atomsas an example. Both steps proceed in remarkably good yields. Thesynthesis is detailed below in Scheme 1:

Scheme 1 shows the synthesis commencing with the formation of theammonium salt, 3. AMPS-oxyethylene ammonium salt monomer, 3, issynthesized by combining 2-acrylamido-2-methyl-1-propanesulfonic acid(AMPS), 1, with an equal molar quantity of freshly distilledtris[2-(2-methoxyethoxy)-ethyl]amine, 2, under an inert atmosphere,including but not limited to nitrogen or argon. The mixture is stirredfor 8 hours at ambient temperature until the AMPS crystals arecompletely dissolved and converted into the monomer salt, 3, a slightlyyellow viscous clear liquid. With no further purification necessary, aradical initiator such as 2,2′-azobisisobutyronitrile (AIBN) is added tothe reaction flask utilizing air-free handling techniques under an inertatmosphere. The reaction mixture is heated to 70° C. and reacted for 18hrs. The transparent amber ionic liquid polymer salt, 4, may be used asis or further purified to remove a small percentage (<5%) of unreactedmonomer by dissolving in acetone and reprecipitating in cold diethylether. This precipitate is collected by cold suction filtration and uponwarming, becomes a tacky transparent yellow liquid. The glass transitiontemperatures of the oxyethylene amine, 2, AMPS monomer salt, 3, andpolymer ionic liquid, 4, are −104, −57, and −49° C. respectively.Infrared, and NMR spectral analyses—are consistent with the polymerrepeat unit structure. The intrinsic viscosity value of 0.3 isindicative of a low, but significant molecular weight. Electrowettingcharacterization displays a reversible contact angle change from 75° to30° with an applied voltage change from 0 to 157 volts. This change islarger than that of the monomer salt or of commercial ionic liquids.Those skilled in the art would understand that other radical initiatorscan be used, and the time and temperature of the reaction would varydepending upon the initiator chosen.

The properties of this ionic liquid polymer salt can be tuned by varyingthe identity, size and symmetry of the substituents on the amine counterion or by varying the structure and function of the sulfonated monomer;such that, the monomer itself could go from being a repeat unit to abranch point to a crosslink.

These and other features and advantages of the present invention will bepresented in more detail in the following specification of the inventionand the accompanying figures, which illustrate, by way of multipleexamples, the principles of the invention.

Example

The formation of the ionic liquid monomer salt and its polymerization isdepicted in Scheme 1. The polymerizable component is the2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) monomer which is acrystalline compound with a 192° C. melting point. It is converted to aliquid ammonium salt by addition of an equimolar quantity oftris[2-(2-methoxyethoxy)-ethyl]amine. This tertiary amine was selectedas one that would solvate the AMPS monomer without the need of a solventand one whose oxyethylene substituents would shield the protonated ioniccenter from coordinating with the sulfonated anion, thereby depressingthe melting point or glass transition to a very low temperature.

The glass transitions of the free amine 2 (−104° C.) and of theAMPS-ammonium salt 3 (−57° C.) presented in the differential scanningcalorimetry (DSC) thermogram in FIG. 1 demonstrate the remarkablecapability of the oxyethylene substituted amine to extend the liquidrange of a salt to low temperatures. After the liquid ammonium sulfonatemonomer formation is complete, a free radical initiator(2,2′-azobisisobutyronitrile, AIBN) is added and reacted at 70° C. Themonomer conversion is 95%, and the product is a clear acetone-solubleviscous liquid. The spectroscopic characterization (IR, ¹H and ¹³C NMR)is consistent with polymer structure depicted in Scheme 1. The molecularweight is low but significant as indicated by an intrinsic viscositymeasurement of 0.30 which is uncorrected for an observed polyelectrolytecoil expansion effect. The glass transition temperature of ionic liquidpolymer 4 is −47° C. as depicted in the FIG. 1 thermogram. This is aremarkably modest increase from that of the unpolymerized monomer salt.

Electrowetting is an electrostatically driven surface effect where aliquid droplet's spreading on a hydrophobic surface is modulated byapplication of a voltage to the droplet and an underlying conductingsubstrate. A schematic of this effect is illustrated in FIG. 2. Thedroplet rests on a very thin low-dielectric insulating film (Teflon AF)which is supported on a conducting substrate and is contacted at the topby a very fine wire contact. Application of a voltage builds up a layerof charge on both sides of the interface with the dielectric film anddecreases the interfacial energy.

The observed response is a spreading of the droplet and a changing ofits curvature. The dependence of the droplet's contact angle, θ, on theapplied voltage, V, is described by the Young-Lippmann equation asfollows (Equation 1):

${\cos\;\theta} = {{{\cos\;\theta_{0}} + {\frac{1}{2\gamma}{CV}^{2}}} = {{\cos\;\theta_{0}} + {\frac{ɛ\; ɛ_{0}}{2\gamma\; t}V^{2}}}}$

where θ₀ is the contact angle at zero voltage, C is the capacitance perunit area, γ is the surface tension of the liquid, ∈ is the permittivityof the insulating dielectric, ∈₀ is the electric constant and t is thethickness of the dielectric layer. Surface tension measurements of themonomer 3 and polymer 4 are 38.1 and 47.0 mJ/m² respectively. Thecontact angle measurements of the AMPS-ammonium salt monomer 3 andpolymer 4 as a function of applied DC voltage are shown in FIG. 3, andphotographs of the ionic liquid polymer droplet at zero and maximumapplied voltages in FIG. 4 show a substantial contact angle change.

A parabolic best fit of the experimental contact angles to the voltagedependence of Equation 1 is represented by the continuous line. Aparabolic best fit of the experimental contact angles to the voltagedependence of Equation 1 is represented by the continuous line in FIG.3. Separate best fits were made for the negative and positive voltageregions of the curves. The electrowetting curve for the AMPS-ammoniumsulfonate monomer, 3, is similar to those reported for small moleculeionic liquids in both magnitude and shape. No evolution of gas bubbles,discoloration, or degradation was observed between 0 and 157 V.

A monomer vs. polymer comparison of contact angle data for AMPS-ammoniumsalt system presents an interesting contrast. The polymer displays alarger contact angle at zero voltage; a similar magnitude of contactangle change over the voltage range; a departure from the Young-Lippmannequation at a lower voltage; and a dissymmetry between the negative andthe positive voltage sections of the electrowetting curve.

These observed differences in electrowetting behavior correlate with thehigher surface tension of the polymer and with the polyelectrolytemolecular structure. When no voltage is applied, the greater surfacetension of the polymer is clearly consistent with the larger contactangle observed. When a voltage is applied, an electric field isconcentrated across the Teflon AF interface and an ionic double layer ofcharge forms in the liquid at this interface. This decreases the surfacetension at the solid-liquid interface and results in spreading of theliquid on the charged surface. A depiction is illustrated in FIG. 2. Thepolymer differs from the monomer in that one of the charged componentsis a small cationic molecule and the other is a large anionicpolyelectrolyte. When the substrate is charged, an oppositely chargedcomponent of the ionic liquid is adsorbed at the Teflon AF-ionic liquidinterface. On reversal of polarity, a grouping of small molecule ionsexchanges position with the polyelectrolyte. This disparity of molecularsize correlates with the dissymmetry observed between the positive andnegative sides of the electrowetting curve in FIG. 3.

All ¹H-HMR spectroscopy were obtained using a Bruker AC-300 spectrometerusing d6-DMSO as solvent. FTIR spectra were obtained using a NicoletMagna-IR 750 spectrometer with sample supported on a NaCl plate under anitrogen purge of 40 cm³ min⁻¹. All differential scanning calorimetric(DSC) analysis were performed on a TA Instruments DSC Q100 Modulatedthermal analyzer at a heating rate of 10° C. min-1 and a nitrogen purgeof 25 cm3min-1.

Synthesis of AMPS-ammonium salt monomer. A nitrogen purged 50 ml schlenkflask was charged with freshly distilledtris[2-(2-methoxyethoxy)-ethyl]amine (1.69 g, 5.24 mmol) and2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) (1.09 g, 5.24 mmol)and stirred at ambient temperature for 8 hr or until completelydissolved. Total dissolution completes formation of the AMPS-ammoniumsalt monomer as a transparent light amber oil in a 99% yield which wasused immediately without further purification.

Synthesis of AMPS-ammonium salt polymer. To the prepared monomer wasadded 2,2′-azobisisobutyronitrile (AIBN) (6.0 mg, 0.04 mmol) under anitrogen purge. The reaction flask was then sealed, allowed to vent to abubbler, and heated to 70° C. for 18 hr. Upon cooling to ambienttemperature, the transparent dark amber mixture was dissolved in acetone(10 ml) and precipitated as white flocculants into cold diethyl ether(50 ml) (dry ice/acetone bath) and quickly collected in a dry ice cooledBuchner funnel via suction filtration. As the material warmed, theflocculants became a transparent amber oil which was then dried undervacuum to remove any residual solvents to yield polymer (95%).

Electrowetting actuation setup and experimentation. Actuation wasmeasured using the following setup as illustrated in FIG. 2: a VCAOptimaXE commercial contact angle instrument fitted with a stage wasused to capture individual data points at various DC voltages. An ITOcoated slide, was spin-coated with Teflon AF (DuPont) at 2000 rpm for 30seconds and heat treated in an oven at 80° C. for 18 hr which producedan insulating film with a thickness of 1.29 μm (determined by a KLAProfilometer) with an intrinsic roughness on the order of 0.5 μm. TheITO slide was coupled to an electrode in an area which was absent ofTeflon AF and a Pt wire with a diameter of 0.25 mm was used as thecorresponding top electrode. Small aliquots of ionic liquid monomer (3)or polymer (4) (˜50 μl) were placed onto the ITO/Teflon AF coated slidein droplet form. The Pt electrode was inserted into the droplet andsnapshots were taken at ˜8 V intervals from 0 to 157 V (DC) withpolarities of the electrodes being reversed to obtain measurementsbetween −157 and 0 V (DC) with respect to the ITO electrode. Contactangle software, from AST Products, Inc. VCA Version 1.90.0.9 forWindows, calculated left and right advancing angles for both the AMPSoxyethylene ammonium salt monomer 3 and polymer 4, which are plottedover a voltage range in FIG. 3. The voltage polarity corresponds to theITO electrode. No discoloration, etching, or insoluble residue wasresultant from these experiments involving ionic liquid 3 and 4 nor wasit observed on the ITO surface itself. Once a voltage of 157 was reached(for either polarity), receding contact angles were recorded as thevoltage was slowly decreased back to 0.

Surface Tension Measurements: Surface tensions for both the monomer (3)and polymer (4) were measured at 23° C. by the pendant drop method usingthe VCA contact angle instrument and Pendant Analysis Software (Version2.22) and found to be 38.1 and 47.0 mJ/m2, respectively; higher thanthose previously reported for other ionic liquids. To further addressthis issue, the surface tensions of structurally relevant compounds areconsidered. The surface tension of the free amine(tris[2-(2-methoxyethoxy)-ethyl)amine, 2 was measured at 32.8 mJ/m2 (23°C.). As a representative of a liquid organic sulfonic acid, methanesulfonic acid has a surface tension of 50.2 mJ/m2 [Lange's Handbook ofChemistry, 15th Ed., J. A. Dean, 1999]. A complex formed between thesetwo components would probably have an intermediate value assuming littlecontribution from the ammonium sulfonate salt formation. As such thisammonium-hydrogen-sulfonate ionic liquid is somewhat different fromother ionic liquids including those for which surface tensions have beenreported in that significant hydrogen bonding is incorporated into it.We would speculate that the hydrogen bonding would elevate the surfacetension. We would further speculate that transformation of theAMPS-ammonium salt monomer 3 to polymer 4 would enhance the density ofthe hydrogen bonding and further elevate the surface tension asobserved. Hence, we speculate that a more dense structure of the polymerand hydrogen bonding is consistent with an increased surface tension.

This example represents synthesis and electrowetting of individualmembers of new ionic liquid monomer and polymer systems. The uniquenessof the oxyethylene amine formation of the ammonium cationic speciescontributes to both the ionic and liquid nature of the monomer andpolymer. Even more remarkable is the ability of this polymer to maintainits liquid nature after polymerization and wet a substrate, showingpreference for one polarity based upon the makeup of the ionic backboneof the polymer formed.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1. An ionic liquid monomer salt having the formula:

wherein R, R′, and/or R″ are each independently selected from H, or—(CH₂)_(n)CH₃ where n=1 to 12, or —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or—(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or —(CF₂)_(n)CF₃,—CH₂(CF₂)_(n)CF₃ where n=1 to
 8. 2. A method for making an ionic liquidmonomer salt comprising: mixing an equimolar amount of a sulfonic acidfunctionalized monomer having the formula:

with the organic amine base having the general formula:

wherein R, R′, and/or R″ are each independently selected from H, or—(CH₂)_(n)CH₃ where n=1 to 12, or —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or—(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or —(CF₂)_(n)CF₃,—CH₂(CF₂)_(n)CF₃ where n=1 to 8; and stiffing said mixture under aninert atmosphere until said sulfonic acid functionalized monomer isdissolved, wherein the dissolution completes formation of the ionicliquid monomer salt.
 3. A method for making an ionic liquid polymer saltcomprising: providing the ionic liquid monomer salt claim 1 having thegeneral formula:

wherein R, R′, and/or R″ are each independently selected from H, or—(CH₂)_(n)CH₃ where n=1 to 12, or —(CH₂CH₂O)_(n)CH₃ where n=1 to 4, or—(CH)₃(Si(CH₃)₂O)_(n)CH₃ where n=1 to 6, or —(CF₂)_(n)CF₃,—CH₂(CF₂)_(n)CF₃ where n=1 to 8; adding a free radical polymerizationcatalyst to said ionic liquid monomer salt; stiffing said mixture underan inert atmosphere with application catalyst-specific polymerizationconditions.